Tissue development and function depend upon a complex and dynamic interplay between cells and their surrounding extracellular matrix. Adhesion of cells to extracellular matrix proteins via specific cell surface receptors initiates intracellular signaling events that regulate many key aspects of cell behavior. Moreover, the precise composition and organization of the extracellular matrix contribute to the mechanical and permeability properties of the skin, vasculature, lungs, and other organs. In particular, the mechanical properties of load-bearing tissues, including blood vessels and cardiac muscle, are critical to their performance in the body. Hence, successful approaches to tissue engineering require not only an understanding of how cells interact with and organize extracellular matrix proteins into a complex, three-dimensional structure, but also an understanding of how these events subsequently contribute to the mechanical properties of artificial tissue constructs. Both the actin cytoskeleton and the extracellular matrix contribute quantitatively to the mechanical properties of artificial tissues. Our data indicate that the polymerization of fibronectin into the extracellular matrix plays a unique role in regulating both cytoskeletal tension generation and extracellular matrix organization. Our preliminary studies also indicate that the addition of fibronectin to cell-populated collagen lattices increases the toughness and ultimate strength of these biogels. In this proposal, we will determine how the polymerization and subsequent stabilization of fibronectin into the extracellular matrix contributes to the tensile mechanical properties of an established tissue model. Quantitatively determining how matrix fibronectin affects the tensile properties of collagen-based biogels will provide essential information that ultimately may be used to molecularly tailor extracellular matrices to enhance the performance of load-bearing tissue constructs.